I don't know how the Google ads work, but I believe most of them pay you by the click. I doubt the ads on this site now get very many clicks. Most people come here and return here because this place has the biggest and best selection of desktop wallpapers. Why then would anyone go off to inferior wallpaper sites in search of a good background? If the ads were of a different topic they'd receive more clicks.
I could influence the ads by changing the keywords in the title and in other places, however (if successfull) it would screw up my google ranking in the wallpaper catagory. Right now caedes.net is #1 for "desktop wallpaper" I think. The bottom line is that the site's content is not a very good one for advertisement revenue. I'd bet that the only ads which make much money for the site are one's selling graphics software. It ok though since we are still able to pay for the site's hosting expenses.
After a few minutes surfing us, maybe they find out we just don't have enough Britney photos. Google also makes me wonder how we got the ad for the colitis foundation? ;)